The present invention relates generally to optical fibers and amplifiers, and in particular to narrow-linewidth high-power continuous wave (CW) or quasi-continuous fiber lasers and amplifiers.
Stimulated Brillouin scattering (SBS) is a limiting factor in the evolution of fiber lasers and amplifiers towards higher power. It is also harmful to optical communications signals. Extensive work has been done to mitigate this phenomenon since it was first observed in optical fibers in 1972. A quantum mechanical approach indicates that light quanta (photons) are scattered by acoustic medium excitations (phonons) which are generated through the process of electrostriction. This creates scattered photons at the so-called Stokes frequency. From a semi-classical viewpoint, a nonlinear interaction occurs between the laser and Stokes optical fields through an acoustic wave. It is well-known in the art that once a certain amount of optical power is coupled into or is generated in the fiber, the backscattered Stokes light causes the performance of the fiber to degrade.
The SBS process is characterized by a gain spectrum that determines the SBS response of the medium to the pump frequency. Measurements in silica fibers have established a Brillouin shift of approximately 16 GHz and a linewidth, ΔvB, of approximately 40 MHz at a wavelength of 1064 nm. For laser pulses of width smaller than the phonon lifetime, the SBS process is insignificant as compared to another nonlinear process called stimulated Raman scattering (SRS). Even for continuous wave (CW) pumping, the Brillouin gain is reduced considerably if the linewidth of the pump is greater than ΔvB. However, some applications including directed energy applications require the use of high power narrow linewidth optical fiber amplifiers and lasers and the mitigation of SBS effects will be necessary for the development of kilowatt level devices.
The SBS threshold can be increased by decreasing the effective length of the fiber, increasing the effective area, or somehow manipulating the Brillouin gain in the fiber. The increase in the SBS threshold through the decrease of length is limited by the gain requirements in the fiber. Increased gain per unit length through higher concentrations of rare earth elements is beset with problems associated with a process known as photo-darkening and also by solubility limits. Much work has been done to increase the effective area of the fiber through the use of large mode area (LMA) fibers. While to conventional LMA fiber designs have been successfully pushing the power output of laser amplifiers beyond 100 watts, there is general agreement that new approaches are required for further enhancement of the power output.
A variety of experimental efforts have been attempted or proposed to reduce the SBS threshold through the manipulation of the fiber optical or acoustical properties as related to the overall SBS gain. In U.S. Pat. No. 5,851,259 by Clayton et al., the SBS threshold is reduced by introducing a modulation in the tension applied to the fiber during the draw process. This idea was expanded on in U.S. Pat. No. 6,542,683 by Evans et al. as a permanent, non-uniform stress is imparted to the fiber core through non-uniform thermal expansion and viscosity profiles. The latter inventor shows that a simple modulation of tension during the draw process leads to a marginal increase in the SBS threshold. The technique is limited by the fact that a change in the draw tension leads to a change in the fiber diameter. The latter inventor did not envision a fiber which could be manufactured with polarization maintaining properties.
Thermal frequency shift through a thermal gradient in the fiber is also possible. This frequency shift was measured by Imai et al. to be of the order of a several MHz per degree Kelvin as reported in a 1993 paper in IEEE Photon. Technol. Lett. 16, pp. 133-1337. Along these lines, in the 2007 paper by Jeong et al. in IEEE J. of Selected Topics in Quantum Electron., pp. 546-551, thermal broadening by quantum-defect heating increased the SBS threshold sufficiently high that an approximately 400 Watt polarization maintaining narrow linewidth single mode output was obtained.
In U.S. Pat. Nos. 6,587,623 by Papen et al. and 6,856,740 by Balestra et al. designs that guide the optical signal in the core of the fiber while at the same time guiding the acoustic wave in the cladding are proposed. In the 2006 paper by Wang et al., Proc. of SPIE, 6351, 635109 pp. 1-7, a reduction in the overlap between the optical and acoustic fields is achieved through the fabrication of an aluminum/germanium counter-graded fiber-core composition profile. The result of this design is a continuous change of the acoustic velocity in the fiber core in the radial direction. Unfortunately this technique requires a large acoustic index of refraction gradient which is difficult to implement. Furthermore, it is still unclear whether this technique can be implemented in conjunction with a longitudinal thermal gradient
None of the ideas outlined above have, however, shown great promise in mitigating the SBS effect sufficiently enough so that multi-kilowatt level narrow linewidth fiber lasers and amplifiers can be built. Accordingly, there remains a need for novel techniques for mitigating the SBS effect.